This program of research has two aims: (1) To describe comprehensively the brain mechanisms underlying lordosis in the mating behavior of the female rat, including the effects of estrogen on these mechanisms; and (2) To test the generality of principles regarding steroid sex hormone binding in the brain which, to date, have been derived from studies with rats. 1. We will continue and extend our systematic program of research which links preoptic and hypothalamic mechanisms involved in sexual motivation to lower brainstem and spinal neural pathways which process sensory information and motor outflow in lordosis behavior. We hope thereby to explain and interpret estrogen-sensitive hypothalamic mechanisms in terms of their effects on long "reflex loops" which exert supraspinal control over lordosis. Involved in this program of research are increasingly detailed descriptions of the compound lordosis reflex, by movies and X-ray, and descriptions of sensory and muscular mechanisms. These are to specify as completely as possible those behavioral and peripheral phenomena which we must explain by central neural mechanisms. In turn, spinal and supraspinal lordosis control pathways will be analyzed by coordinated use of transection, lesion, electrical stimulation and single unit recording methods. 2. We will use the autoradiographic approach which has proven successful for studying steroid binding in rat brain to detect and characterize estradiol-concentrating and testosterone-concentrating neurons in a range of vertebrate forms, each selected for its own interest and advantages. With this approach, common vertebrate patterns and/or interesting phylogenetic trends in sex hormone-sensitive neural systems can be revealed.